Silica reinforced rubber composition

ABSTRACT

The present invention relates to a silica reinforced rubber composition and pneumatic tires having treads comprised of the silica reinforced rubber compositions. The silica reinforced rubber composition comprises an elastomer, silica, optionally carbon black and a silica coupler of the formula: ##STR1## wherein R 1 , R 2  and R 3  individually represent hydrogen or an alkyl of 1 to 6 carbon atoms; X represents oxygen or sulfur; and Z represents a hydroxyalkyl group of 2 to 7 carbon atoms.

This is a continuation of application Ser. No. 08/304,294, filed on Sep.12, 1994, now abandoned.

FIELD

This invention relates to rubber compositions which are quantitativelyreinforced with silica. In one aspect, the rubber composition iscomprised of rubber, particularly sulfur cured rubber, reinforced with acombination of silica and specified coupling agent and, optionally,carbon black. The silica reinforced rubber composition is particularlysuited for use in a tread of a pneumatic tire.

BACKGROUND

For various applications utilizing rubber which requires high strengthand abrasion resistance, particularly applications such as tires andvarious industrial products, sulfur cured rubber is utilized whichcontains substantial amounts of reinforcing fillers. Carbon black iscommonly used for such purpose and normally provides or enhances goodphysical properties for the sulfur cured rubber. Particulate silica isalso often used for such purpose, particularly when the silica is usedin conjunction with a coupling agent. In some cases, a combination ofsilica and carbon black is utilized for reinforcing fillers for variousrubber products, including treads for tires.

It is important to appreciate that, conventionally, carbon black is aconsiderably more effective reinforcing filler for rubber products, andparticularly for rubber tire treads than silica if the silica is usedwithout a coupling agent, or silica coupler as it may be sometimesreferred to herein.

Indeed, at least as compared to carbon black, there tends to be a lackof, or at least an insufficient degree of, physical and/or chemicalbonding between the silica particles and the rubber elastomers to enablethe silica to become a sufficient reinforcing filler for the rubber formost purposes, including tire treads, if the silica is used without acoupler. While various treatments and procedures have been devised toovercome such deficiencies compounds capable of reacting with both thesilica surface and the rubber elastomer molecule, generally known tothose skilled in such art as coupling agents, are often used. Suchcoupling agents may, for example, be premixed, or pre-reacted, with thesilica particles or added to the rubber mix during the rubber/silicaprocessing, or mixing, stage. If the coupling agent and silica are addedseparately to the rubber mix during the rubber/silica mixing, orprocessing stage, it is considered that the coupling agent then combinesin situ with the silica.

In particular, such coupling agents are generally composed of a silanewhich has a constituent component, or moiety, (the silane portion)capable of reacting with the silica surface and, also, a constituentcomponent, or moiety, capable of reacting with the rubber, particularlya sulfur vulcanizable rubber which contains carbon-to-carbon doublebonds, or unsaturation. In this manner, then the coupler may act as aconnecting bridge between the silica and the rubber and thereby enhancethe rubber reinforcement aspect of the silica.

In one aspect, the silane of the coupling agent apparently forms a bondto the silica surface, possibly through hydrolysis, and the rubberreactive component of the coupling agent combines with the rubberitself. Usually the rubber reactive component of the coupler istemperature sensitive and tends to combine with the rubber during thefinal and higher temperature sulfur vulcanization stage and, thus,subsequent to the rubber/silica/coupler mixing stage and, therefore,after the silane group of the coupler has combined with the silica.However, partly because of typical temperature sensitivity of thecoupler, some degree of combination, or bonding, may occur between therubber-reactive component of the coupler and the rubber during aninitial rubber/silica/coupler mixing stages and, thus, prior to asubsequent vulcanization stage.

The rubber-reactive group component of the coupler may be, for example,one or more of groups such as mercapto, amino, vinyl, epoxy, and sulfurgroups, preferably a sulfur or mercapto moiety and more preferablysulfur.

Numerous coupling agents are taught for use in combining silica andrubber, such as, for example, silane coupling agents containing apolysulfide component, or structure, such asbis-(3-triethoxysilylpropyl)tetrasulfide and/or polyisoprene rubbertogether with a mixture of silica and carbon black, with silica beingrequired to be a major component of the silica/carbon black reinforcingfiller.

Other U.S. patents relating to silicas and silica reinforced -tiretreads include U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537;3,884,285; 3,938,574; 4,482,663; 4,590,052; 5,089,554 and British1,424,503.

U.S. Pat. No. 4,310,512 discloses various derivatives of acetic andpropionic acid as malodor counteractants. One example of such derivativeis 2-hydroxyethyl phenoxyacetate.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a rubbercomposition is disclosed which comprises (A) 100 parts by weight of atleast one diene-based elastomer, (B) about 10 to about 250 phrparticulate silica and (C) 0.5 to 50 phr of a silica coupler of theformula: ##STR2## wherein R¹, R² and R³ individually represent hydrogenor an alkyl of 1 to 6 carbon atoms; X represents oxygen or sulfur; and Zrepresents a hydroxyalkyl group of 2 to 7 carbon atoms; wherein theweight ratio of said silica coupler to silica is in a range of about1:100 to about 1:5.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with another aspect of the present invention, there isdisclosed a pneumatic tire having a tread comprised of (A) 100 parts byweight of at least one diene-based elastomer, (B) about 10 to about 250phr particulate silica and (C) from 0.5 to 50 phr of a silica coupler ofthe formula: ##STR3## wherein R¹, R² and R³ individually representhydrogen or an alkyl of 1 to 6 carbon atoms; X represents oxygen orsulfur; and Z represents a hydroxyalkyl group of 2 to 7 carbon atoms;wherein the weight ratio of said silica coupler to silica is in a rangeof about 1:100 to about 1:5.

There is also disclosed a process for improving the reinforcingproperties of a rubber polymer containing a siliceous filler comprisingadding from 0.5 to 50 phr of a silica coupler of formula I.

The silica coupling agents are prepared in a known manner by reacting,where X represents oxygen or sulfur, the appropriate aryloxy- orarylthio-acetic acid having the formula: ##STR4## wherein R¹, R², R³ andX have been defined above in formula I with an appropriate polyol,preferably a glycol, or its corresponding alkylene oxide.

Reaction of the appropriate aryloxy- or arylmercapto-acid with theappropriate polyol is conducted in an inert diluent, e.g. benzene,toluene, cyclohexane or xylene, in the presence of an acid catalyst suchas hydrochloric acid, a strong acid ion-exchange resin or toluenesulfonic acid at elevated temperatures, preferably at the refluxtemperature of the mixture. The product is obtained by crystallizationor distillation.

Reaction of the appropriate aryloxy- or arylthio- acid and an alkyleneoxide is conducted in water in the presence of a base such as sodiumhydroxide or potassium hydroxide at a temperature of from about 0°-50°C. Following extraction into an organic solvent, washing to removeunreacted organic acid, drying and removal of the solvent the product isobtained by crystallization or distillation.

Exemplary compounds which may be used to prepare the silica Couplingagents include phenoxyacetic acid, p-isopropylphenoxyacetic acid,o-methylphenoxyacetic acid, p-tert-butylphenoxyacetic acid,3,4-dimethylphenoxyacetic acid and phenylthioacetic acid.

Exemplary polyols which are utilized to prepare the silica couplingagents include ethylene glycol, butylene glycol, 1,2-propanediol,1,3-propanediol, 1,3-butanediol, 1,4-butanediol and 1,6-hexanediol.

An exemplary alkylene oxide is ethylene oxide.

The term "phr" as used herein, and according to conventional practice,refers to "parts by weight of a respective material per 100 parts byweight of rubber, or elastomer".

The amount of silica coupler of formula I in a rubber composition willvary depending on the level of silica that is used. Generally speaking,the amount of the silica coupler of formula I will range from 0.5 to 50.Preferably, the amount will range from 1.5 to 8 phr. Depending on thedesired properties, the weight ratio of the silica coupler of formula Ito silica may vary. Generally speaking, the weight ratio will range from1:100 to 1:5. Preferably, the weight ratio will range from 1:20 to 1:10.

In the description of this invention, the terms "rubber" and "elastomer"may be used interchangeably, unless otherwise prescribed. The terms"rubber composition", "compounded rubber" and "rubber compound" are usedinterchangeably to refer to rubber which has been blended or mixed withvarious ingredients and materials and such terms are well known to thosehaving skill in the rubber mixing or rubber compounding art.

At least one rubber which is used as the silica reinforced rubbercomposition of the present invention is a diene-based elastomer, orrubber. Thus, it is considered that the elastomer is a sulfur curableelastomer. Such elastomer, or rubber, may be selected, for example, fromat least one of cis 1,4-polyisoprene rubber (natural and/or synthetic,and preferably natural rubber), 3,4-polyisoprene rubber,styrene/butadiene copolymer rubbers, styrene/isoprene/butadieneterpolymer rubbers, cis 1,4-polybutadiene rubber andbutadiene/acrylonitrile copolymer rubber.

In one aspect the rubber is preferably of at least two of diene basedrubbers. For example a combination of two or more rubbers is preferredsuch as cis 1,4-polyisoprene rubber (natural or synthetic, althoughnatural is preferred), 3,4-polyisoprene rubber,styrene/isoprene/butadiene rubber, emulsion and solution polymerizationderived styrene/butadiene rubbers, cis 1,4-polybutadiene rubbers andemulsion polymerization prepared butadiene/acrylonitrile copolymers.

In one aspect of this invention, an emulsion polymerization derivedstyrene/butadiene (E-SBR) might be used having a relatively conventionalstyrene content of about 20 to about 28 percent bound styrene or, forsome applications, an E-SBR having a medium to relatively high boundstyrene content, namely a bound styrene content of about 30 to about 45percent.

The relatively high styrene content of about 30 to about 45 for theE-SBR can be considered beneficial for a purpose of enhancing traction,or skid resistance, of the tire tread. The presence of the E-SBR itselfis considered beneficial for a purpose of enhancing processability ofthe uncured elastomer composition mixture, especially in comparison to autilization of a solution polymerization prepared SBR (S-SBR).

By emulsion polymerization prepared E-SBR, it is meant that styrene and1,3-butadiene are copolymerized as an aqueous emulsion. Such are wellknown to those skilled in such art. The bound styrene content can vary,for example, from about 5 to 50%. In one aspect, the E-SBR may alsocontain acrylonitrile to form a terpolymer rubber, as E-SBAR, inamounts, for example, of about 2 to about. 30 weight percent boundacrylonitrile in the terpolymer.

Emulsion polymerization prepared styrene/butadiene/acrylonitrilecopolymer rubbers containing about 2 to about 40 weight percent boundacrylonitrile in the copolymer are also contemplated as diene basedrubbers for use in this invention.

The solution polymerization prepared SBR (S-SBR) typically has a boundstyrene content in a range of about 5 to about 50, preferably about 9 toabout 36, percent. The S-SBR can be conveniently prepared, for example,by organo lithium catalyzation in the presence of an organic hydrocarbonsolvent.

A purpose of using S-SBR is for improved tire rolling resistance as aresult of lower hysteresis when it is used in a tire tread composition.

The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial for apurpose of enhancing the tire's traction when it is used in a tire treadcomposition.

The 3,4-PI and use thereof is more fully described in U.S. Pat. No.5,087,668 which is incorporated herein by reference. The Tg refers tothe glass transition temperature which can conveniently be determined bya differential scanning calorimeter at a heating rate of 10° C. perminute.

The cis 1,4-polybutadiene rubber (BR) is considered to be beneficial fora purpose of enhancing the tire tread's wear, or treadwear. Such BR canbe prepared, for example, by organic solution polymerization of1,3-butadiene. The BR may be conveniently characterized, for example, byhaving at least a 90% cis 1,4-content.

The cis 1,4-polyisoprene and cis 1,4-polyisoprene natural rubber arewell known to those having skill in the rubber art.

The vulcanized rubber composition should contain a sufficient amount ofsilica, and carbon black, if used, to contribute a reasonably highmodulus and high resistance to tear. The silica filler may be added inamounts ranging from 10 to 250 phr. Preferably, the silica is present inan amount ranging from 15 to 80 phr. If carbon black is also present,the amount of carbon black, if used, may vary. Generally speaking, theamount of carbon black will vary from 0 to 80 phr. Preferably, theamount of carbon black will range from 0 to 40 phr.

Where the rubber composition contains both silica and carbon black, theweight ratio of silica to carbon black may vary. For example, the weightratio may be as low as 1:5 to a silica to carbon black weight ratio of30:1. Preferably, the weight-ratio of silica to carbon black ranges from1:3 to 5:1. The combined weight of the silica and carbon black, ashereinbefore referenced, may be as low as about 30 phr, but ispreferably from about 45 to about 90 phr.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930) .

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300.

The silica might be expected to have an average ultimate particle size,for example, in the range of 0.01 to 0.05 micron as determined by theelectron microscope, although the silica particles may be even smaller,or possibly larger, in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc.; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3, etc. The PPG Hi-Sil silicas are currently preferred.

It is readily understood by those having skill in the art that therubber composition would be compounded by methods generally known in therubber compounding art, such as mixing the various sulfur-vulcanizableconstituent rubbers with various commonly used additive materials suchas, for example, curing aids, such as sulfur, activators, retarders andaccelerators, processing additives, such as oils, resins includingtackifying resins, silicas, and plasticizers, fillers, pigments, fattyacid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agentsand reinforcing materials such as, for example, carbon black. As knownto those skilled in the art, depending on the intended use of the sulfurvulcanizable and sulfur vulcanized material (rubbers), the additivesmentioned above are selected and commonly used in conventional amounts.Typical amounts of reinforcing type carbon blacks(s), for thisinvention, if used, are hereinbefore set forth. It is to be appreciatedthat the silica coupler may be used in conjunction with a carbon black,namely pre-mixed with a carbon black prior to addition to the rubbercomposition, and such carbon black is to be included in the aforesaidamount of carbon black for the rubber composition formulation. Typicalamounts of tackifier resins, if used, comprise about 0.5. to about 10phr, usually about 1 to about 5 phr. Typical amounts of processing aidscomprise about 1 to about 50 phr. Such processing aids can include, forexample, aromatic, napthenic, and/or paraffinic processing oils. Typicalamounts of antioxidants comprise about 1 to about 5 phr. Representativeantioxidants may be, for example, diphenyl-p-phenylenediamine andothers, such as, for example, those disclosed in the Vanderbilt RubberHandbook (1978), pages 344-346. Typical amounts of antiozonants compriseabout 1 to 5 phr. Typical amounts of fatty acids, if used, which caninclude stearic acid comprise about 0.5 to about 3 phr. Typical amountsof zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxescomprise about 1 to about 5 phr. Often microcrystalline waxes are used.Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typicalpeptizers may be, for example, pentachlorothiophenol anddibenzamidodiphenyl disulfide.

In one aspect of the present invention, the silica reinforced rubbercomposition is sulfur cured or vulcanized.

The vulcanization is conducted in the presence of a sulfur vulcanizingagent. Examples of suitable sulfur vulcanizing agents include elementalsulfur (free sulfur) or sulfur donating vulcanizing agents, for example,an amine disulfide, polymeric polysulfide or sulfur olefin adducts.Preferably, the sulfur vulcanizing agent is elemental sulfur. As knownto those skilled in the art, sulfur vulcanizing agents are used in anamount ranging from about 0.5 to about 4 phr, or even, in somecircumstances, up to about 8 phr, with a range of from about 1.5 toabout 2.5, sometimes from 2 to 2.5, being preferred.

Vulcanization of the rubber composition of the present invention isgenerally carried out at conventional temperatures ranging from about100° C. to 200° C. Preferably, the vulcanization is conducted attemperatures ranging from about 110° C. to 180° C. Any of the usualvulcanization processes may be used such as heating in a press or mold,heating with superheated steam or hot air or in a salt bath.

Accelerators are used to control the time and/or temperature requiredfor vulcanization and to improve the properties of the vulcanizate. Inone embodiment, a single accelerator system may be used, i.e., primaryaccelerator. Conventionally and preferably, a primary accelerator(s) isused in total amounts ranging from about 0.5 to about 4, preferablyabout 0.8 to about 1.5, phr. In another embodiment, combinations of aprimary and a secondary accelerator might be used with the secondaryaccelerator being used in smaller amounts (of about 0.05 to about 3 phr)in order to activate and to improve the properties of the vulcanizate.Combinations of these accelerators might be expected to produce asynergistic effect on the final properties and are somewhat better thanthose produced by use of either accelerator alone. In addition, delayedaction accelerators may be used which are not affected by normalprocessing temperatures but produce a satisfactory cure at ordinaryvulcanization temperatures. Vulcanization retarders might also be used.Suitable types of accelerators that may be used in the present inventionare amines, disulfides, guanidines, thioureas, thiazoles, thiurams,sulfenamides, dithiocarbamates and xanthates. Preferably, the primaryaccelerator is a sulfenamide. If a second accelerator is used, thesecondary accelerator is preferably a guanidine, dithiocarbamate orthiuram compound. The presence and relative amounts of sulfurvulcanizing agent and accelerator(s) are not considered to be an aspectof this invention which is more primarily directed to the use of silicaas a reinforcing filler in combination with the hydroxyalkylester ofphenoxyacetic acid or of phenylthioacetic acid as a coupling agent.

The silica reinforced rubber compositions of the present invention maycontain a methylene donor. The term "methylene donor." is intended tomean a compound capable of reacting with the silica coupler of formula Iand generate the resin in-situ. Examples of methylene donors which aresuitable for use in the present invention includehexamethylenetetramine, hexaethoxymethylmelamine,hexamethoxymethylmelamine, lauryloxymethylpyridinium chloride,ethoxymethylpyridinium chloride, trioxan hexamethoxymethylmelamine, thehydroxy groups of which may be esterified or partly esterified, andpolymers of formaldehyde such as paraformaldehyde. In addition, themethylene donors may be N-substituted oxymethylmelamines, of the generalformula: ##STR5## wherein X is an alkyl having from 1 to 8 carbon atoms,R⁴, R⁵, R⁶, R⁷ and R⁸ are individually selected from the groupconsisting of hydrogen, an alkyl having from 1 to 8 carbon atoms and thegroup --CH₂ OX. Specific methylene donors includehexakis(methoxymethyl)melamine,N,N',N"-trimethyl/N,N',N"-trimethylolmelamine, hexamethylolmelamine,N,N',N"-dimethylolmelamine, N-methylolmelamine, N,N'-dimethylolmelamine,N,N',N"-tris(methoxymethyl)melamine and N,N',N"-tributyl-N,N',N"-trimethylol-melamine. The N-methylol derivativesof melamine are prepared by known methods.

The amount of methylene donor that is present in the rubber stock mayvary. Typically, the amount of methylene donor that is present willrange from about 0.1 phr to 10.0 phr. Preferably, the amount ofmethylene donor ranges from about 2.0 phr to 5.0 phr.

The weight ratio of methylene donor to the silica coupler of formula Imay vary. Generally speaking, the weight ratio will range from about1:10 to about 10:1. Preferably, the weight ratio ranges from about 1:3to 3:1.

The presence and relative amounts of the above-conventional additivesare not considered to be an aspect of the present invention which ismore primarily directed to the utilization of specified blends ofrubbers in rubber compositions, in combination with silica and thesilica coupler of formula I, as well as, optionally, carbon black, forthe reinforcement of the rubber.

The mixing of the rubber composition can be accomplished by methodsknown to those having skill in the rubber mixing art. For example theingredients are typically mixed in at least two stages, namely at leastone non-productive stage followed by a productive mix stage. The finalcuratives are typically mixed in the final stage which is conventionallycalled the "productive" mix stage in which the mixing typically occursat a temperature, or ultimate temperature, lower than the mixtemperature(s) than the preceding non-productive mix stage(s). Therubber, silica and silica coupler, and carbon black if used, are mixedin one or more non-productive mix stages. The terms "non-productive" and"productive" mix stages are well known to those having skill in therubber mixing art.

The rubber composition of this invention can be used for variouspurposes. For example, it can be used for various tire compounds. Suchtires can be built, shaped, molded and cured by various methods whichare known and will be readily apparent to those having skill in suchart. Preferably, the rubber composition is used in the tread of a tire.As can be appreciated, the tire may be a passenger tire, aircraft tire,truck tire and the like. Preferably, the tire is a passenger tire. Thetire may also be a radial or bias, with a radial tire being preferred.

The invention may be better understood by reference to the followingexamples in which the parts and percentages are by weight unlessotherwise indicated.

The following examples are presented in order to illustrate but notlimit the present invention.

Cure properties were determined using a Monsanto oscillating discrheometer which was operated at a temperature of 150° C. and at afrequency of 11 hertz. A description of oscillating disc rheometers canbe found in the Vanderbilt Rubber Handbook edited by Robert O. Ohm(Norwalk, Conn., R. T. Vanderbilt Company, Inc., 1990), pages 554-557.The use of this cure meter and standardized values read from the curveare specified in ASTM D-2084. A typical cure curve obtained on anoscillating disc rheometer is shown on page 555 of the 1990 edition ofthe Vanderbilt Rubber Handbook.

In such an oscillating disc rheometer, compounded rubber samples aresubjected to an oscillating shearing action of constant amplitude. Thetorque of the oscillating disc embedded in the stock that is beingtested that is required to oscillate the rotor at the vulcanizationtemperature is measured. The values obtained using this cure test arevery significant since changes in the rubber or the compounding recipeare very readily detected. It is obvious that it is normallyadvantageous to have a fast cure rate.

The following tables report cure properties that were determined fromcure curves that were obtained from the rubber stocks that wereprepared. These properties include a torque minimum (Min. Torque), atorque maximum (Max. Torque), minutes to 90% of the torque increase(t90) and difference between the maximum torque and minimum torque(delta torque).

Shore Hardness was determined in accordance with ASTM D-1415.

EXAMPLE 1 Preparation of 2-Hydroxyethyl Phenoxyacetate

A 2-liter, 3-neck, round-bottom flask was equipped with a heatingjacket, thermocouple and Dean-Stark apparatus for water removal. Theflask was charged with 152 g (1.0 mole) phenoxyacetic acid, 62 g (1.0mole) ethylene glycol, 22 g p-toluenesulfonic acid, 260 ml mixed xylenesand swept with nitrogen and sealed under a nitrogen balloon. The flaskcontents were heated to a pot temperature of 150° C. for 1/2 hourwherein 23 ml of water were collected and the reaction system cooled.Volatiles were removed at 110° C. in 4 mm of Hg vacuum to give a waxywhite solid, IR shows formation of ester and disappearance of acidcarbonyl, and mass spectrometry shows the molecular weight of 196.

EXAMPLE 2

In this example, 2-hydroxyethyl phenoxyacetate (prepared in Example 1)was evaluated as an alternative for a commonly used silica, couplingagent, bis-(3-triethoxysilylpropyl) tetrasulfide, in a silica reinforcedrubber composition. Rubber compositions containing the materials set outin Tables 1 and 2 were prepared in a BR Banbury mixer using threeseparate stages of addition (mixing), namely, two non-productive mixstages and one productive mix stage to temperatures of 160° C., 160° C.and 120° C. and times of 4 minutes, 4 minutes and 2 minutes,respectively. The amount of coupler is listed as being "variable" inTable 1 and is more specifically set forth in Table 2.

The rubber compositions are identified herein as Samples 1, 2 and 3,with Samples 2 and 3 utilizing the silica couplers, respectively, andSample 1 considered herein as being a control Without the use of asilica coupler.

The Samples were cured at about 150° C. for about 36 minutes.

Table 2 illustrates the behavior and physical properties of the curedSamples 1-3.

It is clearly evident from the results that a coupling agent is required(Samples 2 and 3) to obtain suitable cured properties in a silicacontaining rubber compound. Such properties include tensile strength atbreak, the 300%/100% modulus ratio, rebound, hardness and Rheovibron E'and tan delta values.

The 2-hydroxyethyl phenoxyacetate used as a silica coupler (Sample 3) isobserved to provide substantial equivalence in these properties as aconventional bis-(3-triethoxysilylpropyl)tetrasulfide silica couplingagent (Sample 2).

This is considered an advantage because it is shown that rubberproperties equivalent to the silane coupler might be achieved with apotential lower cost. Thus, the 2-hydroxyethyl phenoxyacetate isconsidered herein to be a suitable alternative forbis-(3-triethoxysilylpropyl) tetrasulfide as a silica coupling agent ina silica reinforced rubber composition. 2-Hydroxyethyl phenoxyacetatemay be exposed to the atmosphere and adventitious moisture for a muchlonger period of time due to its much greater shelf-life and hydrolyticstability over Si69 which will lose activity on sitting.

When compared with Sample 2, the 2-hydroxyethyl phenoxyacetate showshigher E' and lower Tan delta at 60° C. and less reversion.

                  TABLE 1                                                         ______________________________________                                        1st Non-Productive                                                            Synthetic cis 1,4-Polyisoprene                                                                    100.00                                                    Carbon Black        35.00                                                     Processing Oil      5.00                                                      Zinc Oxide          5.00                                                      Fatty Acid          2.00                                                      Antioxidant.sup.2   2.00                                                      2nd Non-Productive                                                            Silica.sup.3        15.00                                                     Bis-(3-triethoxysilylpropyl)                                                                      variable                                                  tetrasulfide.sup.4                                                            2-Hydroxyethyl Phenoxyacetate                                                                     variable                                                  Productive                                                                    Sulfur              1.40                                                      Accelerator, sulfenamide type                                                                     1.00                                                      ______________________________________                                         .sup.1) commercially available from The Goodyear Tire & Rubber Company        under the designation Natsyn ® 2200;                                      .sup.2) of the polymerized 1,2dihydro-2,2,4-trimethyldihydroquinoline         type;                                                                         .sup.3) Silica obtained as HiSil-210 from PPG Industries, Inc.;               .sup.4) obtained as bis(3-triethoxysilylpropyl)tetrasulfide, commercially     available as Si69 from Degussa GmbH which is provided in a 50/50 blend        with carbon black and, thus, considered as being 50% active when the blen     is considered.                                                           

                  TABLE 2                                                         ______________________________________                                        Sample #       1          2       3                                           ______________________________________                                        Bis- (3-       0          3.0     0                                           triethoxysilylpropyl)                                                         tetrasulfide (50%                                                             active)                                                                       2-hydroxyethyl 0          0       2.0                                         Phenoxyacetate                                                                Rheometer (150° C.)                                                    Delta Torque   22.0       28.4    26.9                                        T.sub.90, minutes                                                                            21.8       18.8    19.5                                        Stress-Strain                                                                 Tensile Strength, MPa                                                                        16.1       19.1    19.8                                        Elongation at Break, %                                                                       648        579     610                                         300%/100% Modulus                                                                            4.42       4.75    4.65                                        Ratio                                                                         Rebound                                                                       100° C., %                                                                            56.4       60.2    60.5                                        Room Temperature                                                                             46.0       47.3    43.7                                        Hardness                                                                      100° C. 42.9       52.8    50.3                                        Room Temperature                                                                             45.9       56.5    57.2                                        Rheovibron                                                                    E' at 60° C., MPa                                                                     7.7        10.9    12.9                                        Tan Delta at 60° C.                                                                   0.126      0.102   0.095                                       Reversion                                                                     60' (pt drop on                                                                              3.0        1.3     0.7                                         Rheometer)                                                                    ______________________________________                                    

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

What is claimed is:
 1. A pneumatic tire having a tread comprised of (A) 100 parts by weight of at least one diene-based elastomer, (B) about 10 to about 250 phr particulate silica and (C) from 0.5 to 50 phr of a silica coupler of the formula: ##STR6## wherein R¹, R² and R³ individually represent hydrogen or an alkyl of 1 to 6 carbon atoms; X represents oxygen or sulfur; and Z represents a hydroxyalkyl group of 2 to 7 carbon atoms.
 2. The pneumatic tire of claim 1 wherein R¹, R² and R³ are each hydrogen.
 3. The pneumatic tire of claim 1 wherein Z is a hydroxyalkyl group of two to four carbon atoms.
 4. The pneumatic tire of claim 1 wherein R¹, R² and R³ are each hydrogen and Z is a hydroxyalkyl group of two carbon atoms.
 5. The pneumatic tire of claim 1 wherein said silica coupler is present in an amount ranging of from 1.5 to 8 phr.
 6. The pneumatic tire of claim 1 wherein the diene based elastomer is selected from at least one of cis 1,4-polyisoprene rubber, 3,4-polyisoprene rubber, styrene/butadiene copolymer rubbers, styrene/isoprene/butadiene terpolymer rubbers, cis 1,4-polybutadiene rubber, and emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubber and butadiene/acrylonitrile copolymer rubber.
 7. The pneumatic tire of claim 1 wherein the silica is characterized by having a BET surface area in a range of about 40 to about 600 square meters per gram and a dibutylphthalate (DBP) absorption value in a range of about 100 to about
 400. 8. The pneumatic tire of claim 1 wherein X is sulfur. 